THE HELIOLATITUDINAL GRADIENT OF THE SOLAR-WIND DURING SOLAR MINIMUM CONDITIONS MODELED BY EXACT HYDRODYNAMIC SOLUTIONS

The heliolatitudinal dependence of observations of the solar wind macr oscopic quantities such as the averaged proton speed and density is mo delled during solar minimum conditions when the rotational and magneti c axes roughly coincide. Published observations via the technique of i nterplanetary scintillations for the previous two solar cycles were us ed, as well as recent data from the plasma experiment aboard the ULYSS ES spacecraft, which also refer to the declining phase of the present solar cycle. A class of exact, two-dimensional solutions of the full s et of steady HD equations is used which is obtained analytically throu gh a nonlinear separation of the variables. The three parameters which emerge in these solutions are fixed from such observations, as well a s from observations of solar rotation. The solutions are consistent wi th observational inferrences that during solar minimum and the declini ng phase of the solar activity cycle, there is a strong heliolatitudin al gradient in rotation averaged proton speed between about 400 - 800 km s(-1) from equator to pole. This modelling also agrees with previou s findings that the gradient in wind speed with the latitude is offset by a gradient in density such that the mass and momentum flux vary re latively little.